Objects traveling through the atmosphere at high velocity may be subjected to high energy impacts from liquid droplets in the air. These liquid droplets may cause severe erosion of the surface of the object. If the object is a vehicle, this may lead to failure of components such as windows and sensors because they may be formed of materials which are more brittle and weaker than the metals forming the remainder of the vehicle. These weaker materials are often selected for use on these vehicles primarily because of their optical or electromagnetic properties rather than for their resistance to erosion.
Therefore, it is desirable to study the resistance to this type of erosion for different materials having the required electromagnetic properties. A liquid jet impact apparatus was developed by The Cavendish Laboratory, Cambridge, England, for this purpose. The Cavendish Laboratory apparatus is capable of firing a single drop of water with well controlled shape, size and velocity at a sample of material to be evaluated. This permits experimentation to develop a fundamental understanding of the damage mechanisms in the kinds of materials commonly used on windows and sensors of high velocity vehicles. It also permits evaluation of potential improvements in the erosion resistance of these materials.
Referring now to FIG. 1, there is shown a portion of a prior art liquid jet impact apparatus 10. In prior art liquid jet impact apparatus 10 projectile 32 is propelled at a high velocity toward metal block 28 and strikes metal block 28 thereby transferring energy to metal block 28. The energy of this impact is transmitted from energy transfer metal block 28 to steel piston 24 and neoprene gasket 22 which are thereby forced into impact water chamber 20. Impact water chamber 20 contains water.
The energy transmitted from projectile 32 to steel piston 24 forces water within impact water chamber 20 through liquid jet nozzle 14 thereby forcing water droplet 12 to exit nozzle 14 at a high velocity toward a material to be evaluated for resistance to erosion. The proper curvature of the meniscus of high velocity water droplet 12 at the time it exits liquid jet nozzle 14 is important for providing reproducible test results using prior art liquid jet impact apparatus 10.
Referring now to FIGS. 2A,B,C, there are shown cross-sectional representations of water droplets 34, 36, 38 extending from liquid jet nozzle 14. Water droplets 34, 36, 38 are formed by adjusting the pressure against metal block 28 and thereby the pressure in water chamber 20. The pressure against steel piston 24 may be adjusted by adjusting metal block 28 manually. This type of adjustment sometimes resulted in a meniscus on water droplet 12 which was relatively stable but was formed with too large a projection from liquid jet nozzle 14 as shown in FIG. 2A with respect to water droplet 34. This large projection from nozzle 14 resulted in an unsatisfactory water droplet 12 being propelled toward the material being tested.
Alternately, the projection of water droplet 12 from liquid jet nozzle 14 could be too small when the pressure against steel piston 24 was determined by adjusting metal block 28 manually. If water droplet 12 was too small, as shown in FIG. 2B with respect to water droplet 36, it could recede back into liquid jet nozzle 14. When liquid droplet 12 receded into nozzle 14 a spray rather than drop formation was propelled toward the material being tested when prior art: liquid jet impact apparatus 10 was fired. Using manual adjustment it was very difficult to provide a well formed water droplet such as water droplet 38, shown in FIG. 2C, within prior art impact apparatus 10.
Thus, within prior art liquid jet impact system 10 there was no precise control of the shape of the meniscus of water droplet 12 even though it is now believed that the shape of the meniscus of water droplet 12 is important for controlling the shape, size and velocity of droplet 12 when fired. Furthermore, it was not possible in liquid jet impact system 10 to adjust the meniscus between the time that water chamber 20 was closed and the time when projectile 32 was fired toward energy transfer metal block 28. This time period is believed to be critical because water droplet 12 protruding from liquid jet nozzle 14 may recede over a relatively short period of time thereby changing the shape of the meniscus. Additionally, it was very difficult to learn how to use prior art impact apparatus 10.